Milling of Cereals Questions and Answers

Answer: C

The flutes and the speed differential shear the kernel open and scrape endosperm from the bran without grinding the bran itself. Smooth rolls, which crush rather than shear, belong to the reduction passages downstream.

Answer: D

By the reduction passages the endosperm has already been separated from the bran, and the task is simply to grind it finer. Smooth rolls crush rather than shear, which suits that purpose exactly and damages little starch.

Answer: A

A sieve combined with an upward air current separates on density and terminal velocity where size alone cannot, since a bran flake and a semolina particle may pass the same aperture. It is what makes high-grade flour possible.

Answer: B

Correct answer: Flour as a percentage of wheat milled

The extraction rate is the mass of flour obtained expressed as a percentage of the wheat milled - 760 kg of flour from 1000 kg of wheat is a 76 per cent extraction.

It is the central commercial and quality variable of a flour mill. A low extraction gives white, low-ash flour from the innermost endosperm; raising it brings in material closer to the bran, so the flour darkens, its ash content rises and its baking behaviour changes. Wholemeal flour is 100 per cent extraction by definition.

Answer: C

Correct answer: Bran contamination

Ash is the mineral residue left after a flour sample is incinerated, and it is used as an index of bran contamination. The bran and aleurone layers are far richer in minerals than the starchy endosperm, so the more bran a flour contains the higher its ash.

The measure is indirect but reliable and quick, which is why it became the standard trading specification: a patent flour might run at 0.4 per cent ash and a wholemeal at 1.5. It says nothing about moisture, protein or starch damage, which are measured separately.

Answer: D

Correct answer: Remove the oil-rich germ

Maize has an unusually large germ, around 10 to 12 per cent of the kernel and roughly a third oil, so degermination serves two purposes at once. It protects the shelf life of the grits and meal, which would otherwise turn rancid, and it recovers the germ as a valuable feedstock for maize oil.

The operation is done with a degerminator that shears the germ and pericarp away from the endosperm without shattering it, since large intact grits are the premium product. The pericarp is removed alongside, but the germ is the object.

Answer: A

Correct answer: The new surface area created

Rittinger's law holds that the energy required for size reduction is proportional to the new surface area created:

\[ E = K_R \left( \frac{1}{x_2} - \frac{1}{x_1} \right) \]

Since surface area per unit mass varies inversely with particle size, the energy rises steeply as the product gets finer - which is why fine grinding is disproportionately expensive. The law fits fine grinding best; Kick's law, based on the logarithm of the size ratio, fits coarse crushing, and Bond's law the intermediate range.

Answer: B

It suits coarse crushing, where the work depends on the proportional reduction achieved rather than on the surface produced. Bond's law, using the square root of size, sits between Kick and Rittinger in its predictions.

Answer: C

Correct answer: The mass retained on each sieve

A sieve analysis reports the mass retained on each sieve of a nested stack, usually as a percentage of the total charge, from which a cumulative distribution and a mean particle size are derived.

Mass is used because it is what a balance measures directly and what the process cares about. Counting particles would be impractical and would weight a single coarse grain the same as a fine one, and bulk density varies with packing rather than with the size distribution the analysis is meant to characterise.

Answer: D

Correct answer: Increases water absorption

Grinding ruptures some starch granules, and damaged starch absorbs several times its own weight of water where intact granules absorb little. Flour with more damaged starch therefore takes up more water in the dough, which raises the yield of bread from a given quantity of flour.

It is also more readily attacked by amylase, so fermentation proceeds faster. Both effects are useful in moderation and harmful in excess - too much damage gives a slack, sticky dough - so starch damage is a controlled variable rather than simply a defect.